Getting accurate measurements before ordering replacement uPVC windows is one of the most important steps in the whole process, and it’s somewhere we see a lot of homeowners run into trouble when attempting DIY projects. As installers working across Portsmouth and the surrounding areas, we measure windows for a living, and there’s a right way to do it that accounts for tolerances, fitting methods, and the realities of older openings that often aren’t quite as square as they should be. Here’s our step by step guide.
How to Measure for Replacement uPVC Windows
- Measure width at the top, middle, and bottom of the opening, then use the smallest figure
- Measure height at the left, middle, and right of the opening, then use the smallest figure
- Always measure to the brickwork or structural opening, not the existing frame
- Deduct an allowance for fitting tolerance, typically around 10mm on width and height combined
- Check the opening for square by measuring both diagonals
- Note the depth of the opening and any obstructions like sills, trims, or render
- Record measurements in millimetres for accuracy
- If in doubt, arrange a professional survey before ordering
Why Accurate Measuring Matters
uPVC windows are manufactured to order based on the measurements provided, and unlike some products, there’s very little room for adjustment once a frame has been made. If a window is ordered too large, it simply won’t fit into the opening. If it’s ordered too small, you’ll end up with excessive gaps that are difficult to seal properly and can lead to drafts, water ingress, and a poor finish around the frame.
This is why getting the measurements right at the outset matters so much, particularly for replacement windows where you’re working with an existing opening that has its own quirks, whether that’s slightly out of square brickwork, uneven sills, or old frames that have shifted slightly over the years.
Step One: Measure the Width
Start by measuring the width of the opening at three points, the top, the middle, and the bottom. Measure to the brickwork or the structural opening itself, not to the face of the existing frame, as the frame may not sit flush with the brickwork all the way around.
You’ll often find these three measurements differ slightly, sometimes by a centimetre or more, particularly in older properties where the opening has moved slightly over time. Once you have all three figures, use the smallest measurement as your width figure. This ensures the new window will fit even at the narrowest point of the opening.
Step Two: Measure the Height
Next, measure the height of the opening at three points, the left side, the middle, and the right side. Again, measure to the structural opening rather than the existing frame, taking the measurement from the head of the opening down to the cill or the bottom of the opening, depending on how the new window will be fitted.
As with the width, use the smallest of the three height measurements. Combined with your smallest width measurement, this gives you the maximum size the new window can be while still fitting into every part of the opening.
Step Three: Allow for Fitting Tolerance
Once you have your smallest width and height figures, you’ll need to deduct an allowance for fitting tolerance. This is the gap left around the new frame to allow for installation, packing, and sealing with expanding foam or silicone.
As a general guide, we typically work to a tolerance of around 10mm combined across the width, meaning roughly 5mm either side, and a similar allowance on the height. However, this can vary depending on the fitting method, the condition of the opening, and whether additional packers or trims will be used. This is one area where local knowledge really helps, as openings in older Portsmouth properties, particularly Victorian and Edwardian terraces, can have quirks that affect how much tolerance is sensible.
Step Four: Check the Opening for Square
Even if your width and height measurements look straightforward, it’s worth checking whether the opening itself is square. To do this, measure both diagonals of the opening, from the top left corner to the bottom right corner, and from the top right corner to the bottom left corner.
If these two diagonal measurements are the same, the opening is square. If they differ, the opening is out of square, which is extremely common in older properties. A window manufactured as a perfect rectangle won’t fit neatly into an opening that isn’t, so this measurement helps determine whether additional allowances or a different fitting approach will be needed, and it’s information worth passing on to whoever is supplying or fitting the windows.
Step Five: Note the Depth and Any Obstructions
Beyond width and height, the depth of the opening matters too, particularly for how the new frame will sit in relation to the external brickwork, any existing trims, render, or cills. Measure the depth from the front face of the opening to the back, and make a note of anything that might affect fitting, such as:
- Existing window boards or cills that may need replacing or trimming
- External render or cladding that overlaps the opening
- Internal plaster or coving close to the frame
- Pipework, alarm sensors, or cabling running near the opening
- Trickle vents or airbricks that need to be retained or repositioned
These details might not change your core measurements, but they’re important for planning the installation itself and avoiding surprises on the day.
Step Six: Record Everything in Millimetres
When recording your measurements, always work in millimetres rather than centimetres or inches. uPVC windows are manufactured to very precise tolerances, often within a millimetre or two, so working in a single consistent unit reduces the chance of errors creeping in through conversion or rounding.
It’s also worth taking a photo of each opening alongside your measurements, and labelling which measurement corresponds to which window if you’re measuring multiple openings. This becomes particularly useful if there’s any back and forth with a supplier or installer before the windows are manufactured.
When to Arrange a Professional Survey
While measuring an opening accurately isn’t complicated in principle, the margin for error with made to order uPVC windows is small, and a mistake can be costly. If you’re working with bay windows, openings that are significantly out of square, properties with non standard construction, or if you’re simply not confident in your measurements, it’s always worth arranging a professional survey before ordering.
A proper survey also gives an installer the opportunity to assess the condition of the opening itself, check for any issues with lintels, cills, or surrounding brickwork that might need attention, and confirm the best fitting method for your specific property. This is particularly relevant for older homes around Portsmouth, where openings have often settled or been altered over the years in ways that aren’t always obvious from a quick look.
Let BJH Windows & Conservatories Handle the Measuring for You
At BJH Windows & Conservatories, we provide free, no obligation surveys for all our replacement uPVC window installations across Portsmouth and the surrounding areas. Our experienced team will measure every opening accurately, account for any quirks in your property’s construction, and ensure your new windows are manufactured to fit perfectly from the outset. Get in touch with us today to arrange your survey and take the guesswork out of replacing your windows.
About the Author
Oscar Hamilton
Trainee ManagerI am the third-generation member of the BJH Windows family! I have a deep-rooted passion for the business. I achieved a First-Class degree in Business Management and Leadership from the University of the West of England. Having worked summers at BJH since my childhood, I have hands-on experience in the industry, even starting as a fitter’s mate. I am a fully qualified FENSA surveyor, combining my academic knowledge and practical expertise to deliver exceptional service, ensuring the family legacy of quality and customer care continues.
